1. Field of the Invention
The present invention relates to a fuel cell stack, and more particularly, to a fuel cell stack in which water discharge ability and water holding property of an oxidizer flow path of a fuel cell unit in the stack are designed depending on the temperature distribution during electrical power generation.
2. Description of the Related Art
The polymer electrolyte fuel cell basically includes a polymer electrolyte membrane having proton conductivity and a pair of catalyst layers and electrodes provided on both surfaces of the polymer electrolyte membrane.
The catalyst layer generally includes a catalyst layer made of platinum or a platinum group metal catalyst. A gas diffusion layer is provided on the outside of the catalyst layer, for supplying a gas and collecting current.
An assembly in which the polymer electrolyte membrane and the catalyst layers are integrated into one is referred to as a membrane electrode assembly (MEA), and a fuel (hydrogen) is supplied to one of the electrodes and an oxidizer (oxygen) is supplied to another electrode to perform power generation in the process of generating water.
The electrode that is supplied with the fuel is called a fuel electrode, and the electrode that is supplied with an oxidizer is called an oxidizer electrode. Electrical power is drawn from the electrodes located on both sides.
Although the theoretical voltage of a fuel cell unit having one membrane electrode assembly is about 1.23 V, there are many cases where a fuel cell unit is driven at about 0.7 V in a normal operation state, and a part of the reaction energy is converted into heat.
Accordingly, in a case where a higher activation voltage is required, a plurality of fuel cell units are stacked, and the respective fuel cell units are electrically connected in series and used.
This type of stack structure is called a fuel cell stack. In the stack, normally, an oxidizer flow path and a fuel path are isolated by a member called as a separator. Each of the plate-shaped separators is provided with a recessed portion and a protruding portion. A recessed portion facing the membrane electrode assembly is used as a gas flow path, and a protruding portion is used as a current collecting portion.
In such a fuel cell stack, a plurality of fuel cell units generate electrical power simultaneously, and the ratio of heat radiation varies depending upon respective portions of the stack because of the stack structure. The heat generated accompanying the electrical power generation is more likely to stay in the fuel cell unit located at the center of the stack, and is more likely to be released from fuel cell units located at either end. Therefore, a temperature distribution is generated in the stack direction such that the temperature is the highest at the center and is relatively lower at both ends. Due to such a temperature distribution, the respective fuel cell units of the fuel cell stack generate electrical power under different temperature conditions.
Therefore, the following inconveniences are likely to occur.
First, flooding is more likely to occur in fuel cell units located at both ends in the stack direction. Flooding is a phenomenon in which water generated at the oxidizer electrode condenses to decrease the gas diffusion of the oxidizer electrode, which degrades fuel cell characteristics. When a temperature distribution is generated in the stack, since water is more likely to condense in fuel cell units having lower temperatures, flooding is more likely to occur in fuel cell units located at the ends.
Furthermore, a dry-out is more likely to occur at the cell units located at the central part in the stacking direction. The dry-out is a phenomenon in which the water content in the polymer electrolyte decreases with the temperature increase, resulting in the internal resistance in the cell unit, which causes a degradation of the fuel cell characteristics.
The dry-out phenomenon is likely to occur at a position where the temperature is sufficiently high so that water generated at the oxidizer electrode does not condense, but is transpired.
In order to eliminate the instability of the characteristics due to such temperature distribution, Japanese Patent Application Laid-Open No. 2005-340173 proposes a fuel cell stack in which there is a distribution in the amount of air provided to each cell unit of a fuel cell stack.
Specifically, the sectional area of an oxidizer flow path formed in a separator of each cell unit, which has a low temperature and is located at an end, is set to be the largest.
By virtue of this structure, the amount of air supplied at both ends becomes large. Therefore, even when the temperature is lower, water hardly condenses, so that the variation in the degree of flooding in the stacking direction is reduced.
Furthermore, Japanese Patent Application Laid-open No. 2004-311279 proposes the following fuel cell. The fuel cell is configured such that the pressure loss of the gas in fuel cell units at both ends of a fuel cell stack is set to be smaller than in the other fuel cell units, whereby a decrease in the ability to discharge generated of water in the fuel cell units at both ends of the stack having lower temperatures and the inconvenience, such as blocking a gas flow path involved therein, can be suppressed.
However, the above-mentioned conventional examples disclosed in Japanese Patent Application Laid-open Nos. 2005-340173 and 2004-311279 have the following problems.
In the fuel cell stacks with configurations disclosed in Japanese Patent Application Laid-open Nos. 2005-340173 and 2004-311279, in order to discharge generated water and to reduce the variation in temperature, it is necessary to generate the flow of an oxidizer with a blower or the like.
More specifically, it is necessary to set auxiliary devices, such as an air circulation mechanism and a blower, and to supply electrical power for driving the auxiliary devices.
Thus, for example, in mobile applications and the like, where the size of the fuel cell system needs to be reduced as much as possible, this creates problems in terms of the size and weight.